Bear, the diminutive stuffed-animal hero who adjusted to having a baby-sitter in Bear and Mrs. Duck, welcomes a cuddly cub sibling into the family in the latest Winthrop/Brewster collaboration. When Nora, Bear's owner, goes off to Grandma's house to get a baby sister for Bear, he imagines he'll be a helpful big brother. But he never expected that ""baby"" Roly-Poly would be larger than himself, or that she would not look at all like him (she is a panda), or that she would cry so much. But Bear's cheerful resolve and special singing voice eventually help to calm things down. Winthrop is again dead-on in addressing toddlers' trepidations about facing unsettling situations. A light and sweet tone works well in a text that continually imparts messages of love, warmth and acceptance without becoming gooey. Brewster's pastel shades and Martha-Stewart-meets-Laura-Ashley decor set the stage for a cozy curl-up tale. In several scenes Roly-Poly's sizable girth is nearly palpable. Ages 4-8. (Mar.)